Active sand seas and the formation of desert loess

Abstract

Loess is a widespread eolian deposit dominated by coarse silt and very fine sand quartz grains (20–80 μm) that serve as an important archive of information on Quaternary climate change. Despite the intensive research during the past century on desert loess formation, its origin remains poorly understood and is still a fundamental problem in sedimentology. The ongoing debate is focused on the first stage of loess formation – the production of coarse silt quartz grains. Although laboratory experiments indicate the potential of dunes with their abundant quartz sand grains as a primary source for generating coarse silt grains, this concept has been generally rejected because field-based evidence of abrasion from sand grains (eolian abrasion) is scarce. Here we have adopted a global view of the sand sea - loess association and examine in detail several well-known late Pleistocene loess regions in different subtropical deserts (Africa, The Middle East and Arabia). This analysis demonstrates that all these loess regions are located downwind of adjacent sand seas. Together with evidence of mineralogical similarity between the loess and the sand dunes and the contemporaneous deposition of the loess and movement of the dunes, these observations suggest that sand seas are an important source for desert loess. Because there is only limited silt storage in sand dunes, we postulate that the silt grains comprising most of the loess are not reworked from the dunes but are generated probably through active eolian abrasion of the sand grains under past climates characterized by intensified winds. As a result, the role of sand dunes and eolian abrasion in the formation of desert loess may be more important than previously thought.